Control apparatus for hydraulic-lift tail gates of vans, and the like

ABSTRACT

The apparatus controls at least one double-acting consumer. A control valve includes at least one valve member movable between first operative, second operative and neutral positions to thereby respectively connect a first fluid-conveying line of the consumer with a supply conduit which is connected to a source of pressurized fluid, with a return conduit which is connected to a tank, and to seal the first line from the conduits. The valve member regulates the flow of fluid in a control conduit for a switch valve. The switch valve has a first port connected to the source of pressurized fluid, a second port connected to a second fluid-conveying line of the consumer, and a third port connected to the return conduit. The switch valve includes at least one valve member normally maintained in a first position establishing communication in the switch valve between the second and third ports thereof and blocking flow through the first port thereof. The valve member of the switch valve is movable under the influence of fluid flow in the control conduit to a second position establishing communication in the switch valve between the first and second ports thereof and blocking flow through the third port thereof.

nited States Patent [191 Herrmann Feb. 11, 1975 CONTROL APPARATUS FORHYDRAULIC-LIFT TAIL GATES OF VANS,

AND THE LIKE [75] Inventor: Walter llerrmann,Vaihingen/Enz,

Germany Robert Bosch, GmbH, Stuttgart. Germany Filed: Nov. 29, 1973Appl. No.: 420,286

[73] Assignee:

[30] Foreign Application Priority Data Dec. 4. 1972 Germany 2259294 [56] References Cited UNITED STATES PATENTS 5/1969 Ruhl 91/411 R X 7/1970Genz 12/1970 Byers 91/411 R Primary ExaminerEdgar W. Geoghegan Attorney,Agent, or FirmMichael S. Striker 57 ABSTRACT The apparatus controls atleast one double-acting consumer. A control valve includes at least onevalve member movable between first operative, second operative andneutral positions to thereby respectively connect a firstfluid-conveying line of the consumer with a supply conduit which isconnected to a source of pressurized fluid, with a return conduit whichis connected to a tank, and to seal the first line from the conduits.The valve member regulates the flow of fluid in a control conduit for aswitch valve. The switch valve has a first port connected to the sourceof pressurized fluid, a second port connected to a secondfluid-conveying line of the consumer, and a third port connected to thereturn conduit. The switch valve includes at least one valve membernormally maintained in a first position establishing communication inthe switch valve between the second and third ports thereof and blockingflow through the first port thereof. The valve member of the switchvalve is movable under the influence of fluid flow in the controlconduit to a second position establishing communication in the switchvalve between the first and second ports thereof and blocking flowthrough the third port thereof.

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SHEET 1 U? 2 :T"T T llll l SEATED- VALVE- MEMBER I TYPE VALVES CONTROLAPPARATUS FOR HYDRAULIC-LIFT TAIL GATES OF VANS, AND THE LIKE BACKGROUNDOF THE INVENTION The invention relates to a hydraulic controlarrangement comprised of at least one valve of the seatedvalve-membertype serving to control a single-acting user device, which in a neutralposition blocks a work conduit leading to a consumer and in twooperative positions connects such work conduit selectively to either afluid supply conduit or to the fluid return conduit and furthermorecontrols the flow of fluid through a control conduit leading from thefluid supply conduit via the multi-way valve to the return conduit, aswitching valve being controlled in dependence upon fluid flow in thecontrol conduit.

A control arrangement is known which makes use of a multi-way valve ofthe seated-valve-member type usually employed to control a single-actingconsumer, and furthermore provided with switch valve control means. Amulti-way valve of the seated-valve-member type, in contrast tofunctionally equivalent valves of the sliding spool type, arecharcterized by extremely low oil leakage losses. However, suchmulti-way valves have the disadvantage that, when employed to controlthe operation of a double-acting user device, two such multiway valvesmust be employed together, with each of the two multi-way valvescontrolling the flow of fluid through only one of the two work conduitsof such double-acting user device. Such a tandem arrangement ofmulti-way valves of the seated-valve-member type is per se known and hasthe disadvantage that the construction if expensive and space-consuming.Also, the activating arrangements of such two multi-way valves must bemade to cooperate with each other.

SUMMARY OF THE INVENTION It is accordingly an object of the invention toprovide a control arrangement provided with a'switch valve and with aleast one multi-way valve of the seated-valvemember type, such as areusually employed to control the operation of a single-acting userdevice, with the control arrangement being of as simple a constructionas is possible for the control of a double-acting user device.Furthermore, the control arrangement should be capable of operating asnearly loss-free as possible.

This object, and others which will become more understandable from thefollowing description, can be met according to one advantageous conceptof the invention by providing a hydraulic control apparatus for at leastone double-acting consumer, such as a hydraulic-lift tail gatearrangement of a truck, moving van, or the like. The apparatus includesat least one control valve comprised of at least one valve membermovable between first operative, second operative and neutral positionsto thereby respectively connect a first fluidconveying line of theconsumer with a supply conduit which is connected to a source ofpressurized fluid, with a return conduit which is connected to a tank,and to seal the first line from the conduits. The valve member regulatesthe flow of fluid in a control conduit for a switch valve. The switchvalve has a first port connected to the source of pressurized fluid, asecond port connected to a second fluid-conveying line of the consumer,and a third port connected to the return conduit. The switch valveincludes at least one valve member normally maintained in a firstposition establishing LII - ports thereof and blocking flow through thethird port thereof.

In this manner it becomes possible to employ at low cost and without anyspecial additional modifications multi-way valves of the typecustomarily employed to control the operation of a single-acting userdevice to instead control the operation of a double-acting user device,in a manner satisfying stringent requirements as to the seal-tightnessof the arrangement. With the concept according to the invention, asuitable control arrangement can be constructed using mass-producedcommercially available units. Also, the control arrangement according tothe invention is characterized by loss-free operation, because when theuser device is to be fed fluid, to actuate the user device in particulardirection thereof, the pressure source, such as a pump,

of the control arrangement is activated only if the nedd for itsoperation actually exists.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description, of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 depicts a first embodiment ofthe invention;

FIG. 2 depicts a portion of a second embodiment of the invention; and

FIG. 3 depicts a portion of a third embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a controlarrangement, generally designated by reference numeral 11. The controlarrangement 11 is operative for controlling the operation of twocylinder-and-piston units generally designated 12 and 13.

The first hydraulic cylinder-and-piston unit 12 is operative forcontrolling the swinging up and down of a (non-illustrated)hydraulic-lift tail gate of a truck, i.e., for controlling the swingingor pivoting down of the hydraulic-lift tail gate to an approximatelyhorizontal orientation and for controlling the swinging up of thehydraulic-lift tail gate to an approximately vertical orientation.

The second hydraulic cylinder-and-piston unit 13 is operative forcontrolling the raising and lowering of the hydraulic-lift tail gate, ina vertical sense. For example, the hydraulic-lift tail gate willinitially be in closed position, with a vertical orientation, perhapsclosing off the rear opening of the cargo compartment of a truck.Firstly, the tail gate will be made to swing downwardly to anapproximately horizontal orientation and the tail gate will be at ahorizontal level substantially flush with the floor of the truck cargocompartment. Thus, cargo in the cargo compartment can be slid or rolledonto the horizontally extending tail gate. The tail gate is then loweredin vertical direction to for example the streel level, so that the cargocan be slide or rolled off the tail gate onto the street surface. Thisdone, the tail gate is then for example caused to rise while maintaininga horizontal orientation, and is thereafter caused to swing upwards backto its initial or closed position.

The control arrangement 1] of FIG. 1 includes a valve block 14 comprisedof a connecting unit 15, a first multi-way valve unit 16, a secondmulti-way valve unit 17 and an end unit 18. The multi-way valves l6, 17are identical and of the seated-valve-member type, as opposed to thesliding-spool type; that is, the blocking of flow effected by valves 16,17, when they do block flow, results from the abutment of a valve memberagainst a valve seat (seated-valve-member type) as opposed to theblocking off of a port by a sliding spool (sliding-spool type). Theimportant difference between valves of the seated-valve-member andsliding-spool types is that the latter type is characterized by asealtightness which is far inferior to the seal-tightness of theseated-valve-member type. Valves l6 and 17 may for example be of thetype disclosed in German Offenlegungschrift l,507,l64, especially inFIG. 6 thereof. A specific construction for the valves 16 and 17 isshown in FIG. of copending and commonly owned US. Pat. application395,327, filed Sept. 7, 1973, of Walter Herrmann et al., the disclosureof which is incorporated herein by reference.

The four-port three-position valves 16 and 17 are connected in parallelwith each other to a pressure fluid supply conduit 19 and to a returnconduit 21. Each multi-way valve l6, 17 has a spring-maintained neutralposition 22 in which it blocks the respective user conduit 23. Eachvalve 16, 17 has a first operative position 24 in which it connects therespective user conduit 23 to the supply conduit 19, and a secondoperative position 25 in which it connects the respective user conduit23 to the return conduit 21.

Furthermore, each multi-way valve l6, 17 is provided with a mechanicalactivating arrangement 26 provided with an associated switching camsurface 27 and a limit switch 28. In the connecting unit there isarranged a hydraulic fuse for the supply conduit 19, in the form of apressure-limiting valve 29. A control conduit 32 leads from a conduit31, which latter is connected to supply conduit 19, to a flow restrictor33, to the valve 16, to the valve 17, to the end unit 18 and there intothe return conduit 21.

The conduit 31 leads to a first port 34 of a switch valve 35, the secondport 36 of which is connected with a second work conduit 37. The thirdport 38 is connected to the retun conduit 21 via a conduit 39. Theswitch valve 35 is provided with a blocking member 41, schematicallydepicted in FIG. 1. The illustrated biasing spring 42 acts upon theblocking member 41 in such a manner as to normally maintain the switchvalve in the position 43 thereof, in which inlet port 43 is blocked andthe second port 36 is connected with the third port 38. The two dashedlines leading from control line 32 to the left and right sides of valve35, respectively, represent the fact that the blocking member 41 issubjected to the pressure prevailing in the respective two portions ofthe control conduit 32. Because of provision of the restrictor 33, thepressure upstream of the restrictor 33 may exceed the pressuredownstream of the restrictor 33 to such an extend as to overcome thebiasing force of biasing spring 42 and move the valve member 41 ofswitch valve 35 to its second position 44, in which it connects thefirst port 34 with the second port 36 and blocks the third port 38.

The supply conduit 19 is supplied with pressure fluid from a pump 45.Pump 45 is driven by an electromotor 46 and pumps pressure medium upfrom a tank 47. A conduit 48 cbnnects the return conduit 21 with thetank 47. A first work conduit 49 connects the user conduit 23 of thevalve 17 with the lower cylinder chamber 51 of the pivoting-controllingunit l2. A first work conduit 52 connects the user conduit 23 of thevalve 16 with the corresponding chamber 53 of the lifting unit 13. Thecylinder chambers 54, of the cylinder-andpiston units 12, 13 areconnected to a common second work conduit 37.

The operation of the arrangement depicted in FIG. I is as follows:

It is initially assumed that the hydraulic-lift tail gate is in theclosed position, i.e., in the position it assumes after being raised invertical direction and then swung upwardly to the earlier defined closedposition thereof.

To cause the tail gate to swing down to a horizontal orientation, thevalve 17 is made to assume its second operative position 25. In thisposition the switching cam surface 27 does not cause the switch member28 to move such a distance as to bridge the respectively associatedelectrical contacts, which when bridged serve to energize the pump drivemotor 46. Accordingly, the pump 45 does not operate and no pressurefluid flows through the control conduit 32. As a result, no fluid forceopposes the biasing force of spring 42 and the valve 35 remains in itsfirst position 43. Accordingly, the chamber 54 of cylinder 12 isconnected with the return conduit 21, via the second work conduit 56,the switch valve 35 and the conduit 39. The hydraulic-lift tail gate canaccordingly swing downwards of its own weight, and as a result pressurefluid will be expelled from the chamber '51 and flow through the workconduit 49 and the valve 17 to the return conduit 21. Simultaneously,pressure fluid in the tank 47 will be sucked up into chamber 54 viaconduit 39, switch valve 25 and conduit 56.

It is should happen that the truck provided with the hydraulic-lift tailgate is parked on an inclined road surface with such an orientation thatthe weight of the tail gate tends to maintain the tail gate in closedposition, instead of tending to cause it to swing down to open position,then the piston of the cylinder 12 remains in its initial position, eventhough the valve 17 is in its second operative position 25. Theswitching cam. surface 27 on the control slide of the multi-way valve 17is advantageously so constructed that it will operate the limit switch28 only after a delay. That is, the person activating the hydraulic-lifttail gate will first attempt to cause the tail gate to swing open underits own weight, by simply moving the valve 17 to its second workposition 25, using the manually operable activator 26. However, in theevent the tail gate does not swing down of its own weight, then theperson activating the tail gate will move the activator 26 of the valve17 all the way to the rightmost end position thereof, causing the switchmember 28 to bridge the associated pair of electrical contacts andenergize the motor 46. As a result, the pump 45 pumps pressure fluidthrough the supply conduit 19, the pressure achieved in the supplyconduit 19 being limited by provision of the pressure-limiting valve 29.A smaller flow of fluid is established along a parallel path through thecontrol conduit 32 to the return conduit 21, since the control conduit32 is not blocked when the valve 16 is in the neutral position 22 and isnot blocked when the valve 17 is in the second operative position 25thereof. The pressure difference which develops upstream and downstreamof the restrictor 33 opposes the biasing force of spring 42 and movesthe valve member 41 of switch valve to the second position 44 thereof.As a result, pressure fluid can flow from the supply conduit 19, throughthe conduit 31, through the switch valve 35 and through the second workconduit 56 into the chamber 54 of the unit 12, so as to develop a forcepositively forcing the tail gate to swing down to open position. Thepressure-limiting valve 29 limits the magnitude of theavailablepressure.

To cause the tail gate to swing upwards back to closed position, theactivator 26 of multi-way valve 17 is moved leftwards to its firstoperative position 24. As a result, the cam surface 27 pushes switch 28closed, turning the motor 46 on immediately. The pump supplies pressurefluid through the-supply conduit 19, through the multi-way valve 17 andthrough the first work conduit 49 into the chamber 51 of the unit 12.When the valve 17 is in the first working position 24, the controlconduit 32 is blocked; as a result, the switch valve 41 assumes itsfirst position 43. Consequently, pressure fluid can freely flow out ofthe chamber 54 of the cylinder 12 through the second work conduit 56,through the switch valve 41, through the conduit 39 and into the returnconduit 21.

The cylinder-and-piston unit 13 can be controlled by the first multi-wayvalve 16 in a manner analogous to the manner in which theunit 12 iscontrolled by the second multi-way valve 17, just discussed. Themultiway valve 16 has a first operative position 24 for effectingraising of the tail gate in vertical direction and has a second workingor operative position 25 for effecting lowering of the tail gate invertical direction. When the tail gate is to be raised in verticaldirection, the pump is always turned on as a result of activation of theswitch 28 of the valve 17. The lowering of the tail gate in verticaldirection occurs almost always in consequence of the tail gates ownweight, and accordingly in such circumstances the pump 45 need not becaused to operate. However, if necessary, the pump 45 can be activated.The cylinder-and-piston unit 13 is a doubleacting unit precisely forsuch reason, and furthermore to make it possible for the tail gate to bepositively pressed down against the surface of a loading ramp, forexample.

In consequence of the parallel connection of the two cylinder-and-pistonunits 12 and 13, the unit 13 can raise the hydraulic-lift tail gate invertical direction while the unit 12 simultaneously causes the tail gateto swing upwardly towards closed position. Likewise, the tail gate canbe caused to simultaneously swing down towards a horizontal orientationwhile being lowered in vertical direction, for example to street level.

FIG. 2 showsan end unit 60, which could replace the connecting unit 15of FIG. 1, to form a second embodiment of the invention. To the extentthat the unit 60 of FIG. 2 is comprised of components identical instructure and function to those in unit 15 of FIG. 1, such componentsare designated in FIG. 2 by the same reference numerals as in FIG. 1.Unit 60 of FIG. 2 differs from unit 15 of FIG. 1 in the followingrespects: A conduit 61 connects the second work conduit 37 and theconduit 39. Provided in conduit 61 is a means 62 for controlling thefluid pressure, here constructed in the form of an adjustablepressure-limiting valve 63. Alternatively, the means 62 could forexample be constructed as an adjustable restrictor.

The operation of a control arrangement in which the unit 60 of FIG. 2replaces the unit 15 of FIG. 1, differs from the operation of the FIG. 1embodiment, in that the pressure of fluid flowing into the chambers 54and 55 of the cylinder-and-piston units 12 and 13 is set by thepressure-limiting valve 63. Since swinging down and lowering of the tailgate requires a lesser force than is needed for raising the tail gateand for causing it to swing upwards, the pressure-limiting valve 63 canbe set for a correspondingly lower pressure. This results in a reducedenergy-consumption requirement.

The means 62 for controlling the fluid pressure can also be provided inthe form of an adjustable restrictor, as mentioned before, if theviscosity and the pressure oscillations in the fluid flow areacceptable. The control arrangement according to FIG. 2 is particularlywell suited for the control of a hydraulic-lift tail gate because of itslow-loss operation and because of the employment of two different workpressures.

FIG. 3 shows a connection unit which can be substituted for theconnection unit 15 of FIG. 1 to form a third embodiment of the controlsystem according to the invention. Identical components in unit 70 ofFIG. 3 and in unit 60 of FIG. 2 are denoted by the same referencenumerals. Unit 70 of FIG. 3 differs from unit 60 of FIG. 2 in thefollowing respects: A switch valve 71 connects the conduits 31, 37 and39. The switch valve 71, in addition to the first position 43 and secondposition 44, has an intermediate third position 72. In the thirdposition 72, the first port 34 is connected with the third port 38, andthe second port 36 is blocked. Furthermore, the switch valve 71 isprovided with a manual activating arrangement 73 having a switching camsurface 74 and an associated end switch 75, the switch 75 when closedserving to energize the pump motor. The means 62 for controlling thepressure of the fluid medium is here provided in the form of anadjustable restrictor 76.

The operation of the FIG. 3 embodiment differs from the operation of theFIG. 2 embodiment in the following respect: When the multi-way valves 16and 17 are in their neutral positions 22 a so-called neutral circulationof fluid is possible, i.e., the pump 45 pumps the largest part of thepressure fluid through the supply conduit 19, through the switch 71,through the conduit 39 in unthrottled manner, and then into thereturn-flow conduit 21. A smaller flow of fluid through the controlconduit 32 serves to maintain the switch valve 71 in its third position72. If the tail gate is to be positively lowered or swung down, i.e.,because the tail is so disposed that its own weight will not result insuch movement, then the manual activating arrangement 73 brings theswitch valve 71 into its second position 44, and the pump 45 willsimultaneously be activated in consequence of the automatic closing ofswitch 75.

The embodiment of FIG. 3 can be resorted to when a continuouslyoperating pump is anyway present so that such a neutral fluidcirculation would be advantageous.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofhydraulic and constructions differing from the type described above.

While the invention has been illustrated and described as embodied in ahydraulic control arrangement for controlling both the swinging movementand pushing vertical movement of a hydraulic-lift tail gate of a van orthe like, it is not intended to be limited to the details shown, sincevarious modifications and structural changes may be made withoutdeparting in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting features thatfrom the standpoint of prior art fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. In a hydraulic control apparatus for at least one double-actingconsumer wherein at least one control valve includes at least one valvemember movable between first operative, second operative and neutralpositions to thereby respectively connect a first fluidconveying line ofsaid consumer with a supply conduit which is connected to a source ofpressurized fluid, with a return conduit which is connected to a tank,and to seal said first line from said conduits, and wherein said valvemember regulates the flow of fluid in a control conduit for a switchvalve, the improvement which consists in said switch valve having afirst port connected to said source of pressurized fluid, a second portconnected to a second fluid-conveying line of said consumer, and a thirdport connected to said return conduit, and wherein said switch valveincludes at least one valve member normally maintained in a firstposition establishing communication in said switch valve between saidsecond and third ports and blocking flow through said first port andmovable under the influence of fluid flow in said control conduit to asecond position establishing communication in said switch valve betweensaid first and second ports and blocking flow through said third port.

2. An apparatus as defined in claim 1, and further ineludingpressure-controlling means having a first port connected to said returnconduit and to said third port of said switch valve and having a secondport connected to said second fluid-conveying line of said consumer andto said second port of said switch valve.

3. An apparatus as defined in claim 2, wherein said pressure-controllingmeans comprises a flow restrictor having two ports constituting saidfirst and second ports of said pressure-controlling means.

4. An apparatus as defined in claim 3, wherein said flow restrictor isadjustable.

5. An apparatus as defined in claim 2, wherein said pressure-controllingmeans comprises a pressurelimiting valve having two ports constitutingsaid first and second ports of said pressure-controlling means.

6. An apparatus as defined in claim 1, and further including apressure-limiting valve connected to said supply conduit for limitingthe pressure therein.

7. An apparatus as defined in claim 1, wherein said switch valve isprovided with biasing means for maintaining said at least one valvemember of said switch valve in said first position thereof, and furtherincluding a flow restrictor in said control conduit for developing apressure difference upstream and downstream of said flow restrictor inresponse to the establishment of flow through said control conduit, andconnected to said control conduit upstream and downstream of said flowrestrictor and operative in response to development of said pressuredifference for applying to said at least one valve member of said switchvalve a force opposing the force of said biasing means and moving saidvalve member of said switch valve to said second position thereof.

8. An apparatus as defined in claim 1, wherein said at least one valvemember of said switch valve has an intermediate third position in whichit establishes fluid communication in said switch valve between thefirst and third ports of the latter and in which it blocks fluid flowthrough said second port of said switch valve.

9. An apparatus as defined in claim 8, wherein said source ofpressurized fluid includes a pump operative for pumping fluid into saidsupply line, and wherein said switch valve is further provided withactivating means for initiating operation of said pump when said atleast one valve member of said switch valve is moved into said secondposition thereof.

10. An apparatus as defined in claim 1, and further including anadditional control valve in addition to said first-mentioned controlvalve, and an additional consumer in addition to said first-mentionedconsumer, and wherein said additional control valve includes at leastone valve member movable between first operative, second operative andneutral positions to thereby respectively connect a respective firstfluid-conveying line of said additional consumer with said supplyconduit, with said return conduit and to seal said respective first lineof said additional consumer from said supply and return conduits, andwherein said valve member of said additional control valve regulates theflow of fluid in said control conduit in cooperation with the regulationof fluid in said control conduit by said valve member of saidfirst-mentioned control valve, and wherein said additional consumer hasa respective second fluidconveying line connected to said second port ofsaid switch valve.

11. An apparatus as defined in claim 1, wherein said source ofpressurized fluid includes a pump operative for pumping fluid into saidsupply line, and wherein said control valve is further provided withactivating means for initiating operation of said pump in automaticresponse to movement of said valve member of said control valve into oneof said operative positions thereof.

12. An apparatus as defined in claim 11, wherein said activating meanscomprises means operative for initiating operation of said pump inautomatic response to movement of said valve member of said controlvalve into said first operative position thereof.

13. An apparatus as defined in claim 12, wherein said activating meanscomprises means operative for selectively initiating operation of saidpump when said valve member of said control valve is in said secondoperative position thereof.

14. An apparatus as defined in claim 1, wherein said consumer is ahydraulic-lift tail gate arrangement.

15. An apparatus as defined in claim 1, wherein said control valve is ofthe seated-valve-member type.

1. In a hydraulic control apparatus for at least one doubleactingconsumer wherein at least one control valve includes at least one valvemember movable between first operative, second operative and neutralpositions to thereby respectively connect a first fluid-conveying lineoF said consumer with a supply conduit which is connected to a source ofpressurized fluid, with a return conduit which is connected to a tank,and to seal said first line from said conduits, and wherein said valvemember regulates the flow of fluid in a control conduit for a switchvalve, the improvement which consists in said switch valve having afirst port connected to said source of pressurized fluid, a second portconnected to a second fluid-conveying line of said consumer, and a thirdport connected to said return conduit, and wherein said switch valveincludes at least one valve member normally maintained in a firstposition establishing communication in said switch valve between saidsecond and third ports and blocking flow through said first port andmovable under the influence of fluid flow in said control conduit to asecond position establishing communication in said switch valve betweensaid first and second ports and blocking flow through said third port.2. An apparatus as defined in claim 1, and further includingpressure-controlling means having a first port connected to said returnconduit and to said third port of said switch valve and having a secondport connected to said second fluid-conveying line of said consumer andto said second port of said switch valve.
 3. An apparatus as defined inclaim 2, wherein said pressure-controlling means comprises a flowrestrictor having two ports constituting said first and second ports ofsaid pressure-controlling means.
 4. An apparatus as defined in claim 3,wherein said flow restrictor is adjustable.
 5. An apparatus as definedin claim 2, wherein said pressure-controlling means comprises apressure-limiting valve having two ports constituting said first andsecond ports of said pressure-controlling means.
 6. An apparatus asdefined in claim 1, and further including a pressure-limiting valveconnected to said supply conduit for limiting the pressure therein. 7.An apparatus as defined in claim 1, wherein said switch valve isprovided with biasing means for maintaining said at least one valvemember of said switch valve in said first position thereof, and furtherincluding a flow restrictor in said control conduit for developing apressure difference upstream and downstream of said flow restrictor inresponse to the establishment of flow through said control conduit, andconnected to said control conduit upstream and downstream of said flowrestrictor and operative in response to development of said pressuredifference for applying to said at least one valve member of said switchvalve a force opposing the force of said biasing means and moving saidvalve member of said switch valve to said second position thereof.
 8. Anapparatus as defined in claim 1, wherein said at least one valve memberof said switch valve has an intermediate third position in which itestablishes fluid communication in said switch valve between the firstand third ports of the latter and in which it blocks fluid flow throughsaid second port of said switch valve.
 9. An apparatus as defined inclaim 8, wherein said source of pressurized fluid includes a pumpoperative for pumping fluid into said supply line, and wherein saidswitch valve is further provided with activating means for initiatingoperation of said pump when said at least one valve member of saidswitch valve is moved into said second position thereof.
 10. Anapparatus as defined in claim 1, and further including an additionalcontrol valve in addition to said first-mentioned control valve, and anadditional consumer in addition to said first-mentioned consumer, andwherein said additional control valve includes at least one valve membermovable between first operative, second operative and neutral positionsto thereby respectively connect a respective first fluid-conveying lineof said additional consumer with said supply conduit, with said returnconduit and to seal said respective first line of said additionalconsumer from said supply anD return conduits, and wherein said valvemember of said additional control valve regulates the flow of fluid insaid control conduit in cooperation with the regulation of fluid in saidcontrol conduit by said valve member of said first-mentioned controlvalve, and wherein said additional consumer has a respective secondfluid-conveying line connected to said second port of said switch valve.11. An apparatus as defined in claim 1, wherein said source ofpressurized fluid includes a pump operative for pumping fluid into saidsupply line, and wherein said control valve is further provided withactivating means for initiating operation of said pump in automaticresponse to movement of said valve member of said control valve into oneof said operative positions thereof.
 12. An apparatus as defined inclaim 11, wherein said activating means comprises means operative forinitiating operation of said pump in automatic response to movement ofsaid valve member of said control valve into said first operativeposition thereof.
 13. An apparatus as defined in claim 12, wherein saidactivating means comprises means operative for selectively initiatingoperation of said pump when said valve member of said control valve isin said second operative position thereof.
 14. An apparatus as definedin claim 1, wherein said consumer is a hydraulic-lift tail gatearrangement.
 15. An apparatus as defined in claim 1, wherein saidcontrol valve is of the seated-valve-member type.